1. Field of the Invention
The present invention relates to a backlight device having a number of light sources, each including a plurality of light-emitting diodes of three colors or more mounted on one surface, for combining lights emitted from the light-emitting diodes into white light, and applying the white light to a rear surface of a display panel, a method of driving the backlight device, and a color image display apparatus incorporating the backlight device.
2. Description of the Related Art
Television receivers have long been CRT (Cathode-Ray Tube) television receivers since the beginning of television broadcasts. In recent years, however, thin television receivers incorporating a liquid crystal display (LCD) panel or a plasma display panel (PDP) have become mainstream television receivers. Particularly, liquid crystal display panels have become more and more popular as they can be energized with low power consumption and the cost of large-size liquid crystal display panels is lower.
Color liquid crystal display panels are mainly of the backlight design including a color liquid crystal panel having color filters combined with a backlight device for supplying illuminating light to the color liquid crystal panel. The backlight device generally employs as a light source a fluorescent lamp such as a cold cathode fluorescent lamp (CCFL) for emitting white light from a fluorescent tube. Since mercury sealed in the fluorescent tube of the CCFL is environmentally harmful, light-emitting diodes (LED) which are of high luminance, lower power consumption, and long life have begun to be used as a desirable light source in place of the CCFL.
Light-emitting diodes which have heretofore been available include red light-emitting diodes for emitting red light and green light-emitting diodes for emitting green light. In addition, blue light-emitting diodes for emitting blue light have just started to be mass-produced, so that three color primaries can now be produced by these red, green, and blue light-emitting diodes.
In backlight devices using light-emitting diodes as the light source, white light of high chromatic purity which is produced as a mixture of red light, green light, and blue light emitted respectively from the red, green, and blue light-emitting diodes is supplied to a color liquid crystal panel to provide a much wider color reproduction range than possible with the conventional CCFL light sources. If light-emitting diodes having light-emitting chips of high output capability are used as light-emitting diodes for light sources, then the backlight device has greatly improved luminance characteristics.
For details of a color liquid crystal display apparatus with red, green, and blue light-emitting diodes, reference should be made to Japanese Patent Laid-open No. 2006-171693.
The CCFL heretofore used in the backlight device for liquid crystal displays has its white color temperature which cannot be set to a desired value by the user because of structural limitations thereof. If the color temperature of a liquid crystal display is to be changed, then the cold cathode-ray tube needs to be replaced or the aperture of the liquid crystal has to be adjusted to achieve a desired white color temperature. However, it takes time and expenses to replace the cold cathode-ray tube, and adjusting the aperture of the liquid crystal to achieve a desired white color temperature results in an elimination of liquid crystal gradations which leads to a reduction in the quality of displayed images.
The liquid crystal display apparatus with red, green, and blue light-emitting diodes being used as the light sources is problematic in that the light emission of the red, green, and blue light-emitting diodes varies due to ambient temperature changes, product variations and aging, bringing the displayed light out of white balance to cause luminance and color hue variations.
Furthermore, the light-emitting diodes have their light emission characteristics suffering temperature drifts and aging even if they are energized with a constant current.